Problem: Simplify the following expression: $y = \dfrac{7q}{8} - \dfrac{7q}{9}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $8$ and $9$ $\lcm(8, 9) = 72$ $ y = \dfrac{9}{9} \cdot \dfrac{7q}{8} - \dfrac{8}{8} \cdot \dfrac{7q}{9} $ $y = \dfrac{63q}{72} - \dfrac{56q}{72}$ $y = \dfrac{63q -56q}{72}$ $y = \dfrac{7q}{72}$